1. Field of the Invention
The present invention relates to a transfer material on which an image is transferred from an image bearing member, for example in an electrophotographic apparatus, and an image forming method for such transfer material.
2. Related Background Art
The conventional electrophotographic apparatus employs charged resin particles, called xe2x80x9ctonerxe2x80x9d, for image formation and obtains a permanent image by developing an electrostatic latent image formed on a photosensitive member with such toner. Then, the thus obtained toner image is transferred onto a transfer material such as paper or film employing an electrostatic field formed by a transfer charger such as a corona charger or a roller charger. The toner image is fixed to the transfer material by applying heat and pressure to the transfer material bearing the transferred toner image.
Therefore, for forming a clear image on the transfer material composed, for example, of a polymer plastic film, it is necessary to apply the above-mentioned electrostatic field in a uniform and efficient manner. For this purpose, the film conventionally used as the transfer material is subjected to an antistatic treatment in order to prevent electrostatic charging-up. Such antistatic treatment of the film is defined by the surface resistivity thereof in the monochromatic copying apparatus. For example, Japanese Patent Publication No. 51-34734 teaches that the appropriate range of the specific surface resistivity of the film subjected to antistatic treatment is 106 to 1016 xcexa9/xe2x96xa1.
Such antistatic treatment also prevents frictional charging of the transfer material, consisting of a polymer film, by contact with other members before it is used for image transfer, thereby avoiding sheet jamming resulting from the electrostatic adhesion of the charged transfer material to other members of the sheet transport path due to charge-up of the transfer material.
However, the transfer material consisting of a polymer film or resin-impregnated paper (paper fibers appear on the paper surface) has a higher electrical resistance in comparison with ordinary paper. In case of color image formation in a multi-color electrophotographic apparatus with such transfer material, the charging which occurs at the transfer of the image of the first color induces uneven charge on the transfer material, which results in an uneven transfer of the images of the second and subsequent colors, or raises a static charge-up due to increase of the surface potential of the transfer material. Accordingly, it becomes difficult to transfer the images of plural colors onto the transfer material.
For avoiding such phenomenon, the specific surface resistivity on both faces of the transfer material is preferably maintained within a range of 106 to 1010 xcexa9/xe2x96xa1 under any environmental condition from a low humidity condition to a high humidity condition. An excessively low surface resistivity below 106 xcexa9/xe2x96xa1 cannot provide an electrostatic field required for image transfer since the charge induced by the transfer charger escapes, while a surface resistivity exceeding 1010 xcexa9/xe2x96xa1 tends to lead to the charging-up of the sheet mentioned above.
However, even when the specific surface resistivity of the transfer materials is maintained within the range of 106 to 1010 xcexa9/xe2x96xa1 by antistatic treatment, conducting image formation with such transfer material generates a line-shaped electrostatic latent image in a portion corresponding to the end of the transfer material, thereby causing a linear image defect in the image formed next.
Particularly, in the image forming apparatus of a system in which the photosensitive member charged to a predetermined polarity is subjected to image exposure with a laser beam and is developed with toner of a charging polarity the same as the above-mentioned predetermined polarity (reversal developing method) to obtain a toner image, as in the recent digital copying machine, the charge given to the transfer material by the transfer charger is of a polarity opposite to the above-mentioned predetermined polarity. Therefore, the photosensitive member is charged in a polarity opposite to the above-mentioned predetermined polarity, and the above-described line-shaped electrostatic latent image cannot be eliminated by simple charge eliminating means.
In order to avoid generation of such line-shaped electrostatic latent image on the surface of the photosensitive member corresponding to the end of the transfer material, the Japanese Patent Laid-Open Application No. 8-202065 discloses a transfer material having different composition on both faces, with a specific surface resistivity of 1011 xcexa9/xe2x96xa1 or more in at least edge portions of at least one of the faces.
However, even if the specific surface resistivity is made equal to 1011 xcexa9/xe2x96xa1 or higher, the transfer material is susceptible to the influence of humidity in case it is resin-impregnated paper or antistatically treated film, so the specific surface resistivity tends to become lower than 1011 xcexa9/xe2x96xa1 even the transfer material is cautiously treated to avoid moisture absorption. For this reason, the formation of the remnant line-shaped electrostatic latent image on the photosensitive member is often unavoidable, thereby leading to the line-shaped image defect on the image obtained next.
The object of the present invention is to provide a transfer material capable of preventing formation of an electrostatic latent image at a portion of the image bearing member corresponding to an end portion of the transfer material.
Other objects of the present invention will become fully apparent from the following detailed description, which is to be taken in conjunction with the attached drawings.